The present invention relates to cellular wireless communications systems generally and more particularly to apparatus and methods for cellular communications with base stations.
Cellular multiple access communications date back to the early eighties. The nineties witnessed an outburst of this type of service throughout the world and the introduction of digital technologies. The market is expected to soar and expand into Personal Communication Services (PCS), offering personal service, a host of value added features, and total personal mobility, indoors and outdoors. Broadband services are expected to emerge at the beginning of the next century. These may require a partial renewal of the network infrastructure.
Cellular mobile communication attempts to provide mobility, multi-user capacity (many independent users access the system), coverage (service is offered over a large contiguous area) and grade and quality of service.
Cellular communications are generally limited by local codes to a range of frequencies. A widely used technique of cellular communications employs spatial isolation in order to be able to reuse the same frequencies beyond a given range called a guard zone. The communications of each user is maintained with a base station, whose antenna is elevated above the scenery in order to achieve a well defined and controlled coverage are. Sectorization is achieved by directive antennas that illuminate only one sector, thereby reducing interference, enhancing performance and reducing a pattern of frequency reuse.
Each sector of cellular communications is characterized by a number of calls per unit area, also called area capacity. Area capacity may be increased by reducing the cell size. Small cells that are positioned below roof tops in urban areas are called microcells. These use lower and smaller antennas. The cell hardware is more compact, and in some cases has less circuits. Another technique for microcells involves the antenna and RF circuitry only, remote from the cell equipment and connected via RF, fiber or microwave link, to the cell. Such an arrangement is especially attractive for operators in possession of RF or fiber trunking, like CATV companies.
A future trend to increase the capacity of large cells involves smart antennas. These are multibeam array antennas at the base stations, controlled to form narrow beams that are matched to the disposition of the desired user and the sources of interference. These are expected to enhance the coverage and the capacity. The complexity involved in this technology is expected to be relieved with new cell architectures, including, among others, active antenna modules.
The network infrastructure of a typical modern cellular communications system includes a number of base stations, the actual number being related to the capacity required (measured in Erlangs, which is the number of fully occupied circuits), and to the coverage area. The base stations generally constitute about 80% of the network cost. A typical cost for a full capacity large cell base station is $500,000-$1,000,000. The infrastructure also includes interconnect trunking, which depends mainly on the total length of interconnect lines, and switching fabric, which depends on the number of cells and calling load (measured in BHCAxe2x80x94Busy Hour Call Attempts). The cost of the basic service of providing airtime depends mainly on the number of base stations and on their cost.
One of the problems of cellular communications systems is transmission losses. The transmit chain of a first generation base station consists of single carrier HPA""s, filtered, combined and relayed by a high power cable to the mast. The losses involved in the chain amount to 8-10 dB. The carrier spacing is restricted by the combiner to at least 600 KHz.
In an effort to cut down losses, second generation base stations were developed. A second generation base station includes a MCLPAxe2x80x94Multi Carrier Linear Power Amplifier. This reduces the losses and adds flexibility to the design of the carriers (frequency allocations). A low noise amplifier (LNA) is used in the receive chain in the base station. The LNA reduces cable losses which degrade the system noise figure. An additional receive antenna is typically used for diversity. Recent installations place the LNA on the mast.
However, the MCLPA is an expensive part, running from $10,000 for a minicell to over $100,000 for a full capacity cell. Furthermore, MCLPA""s are currently supplied to the whole market by a limited number of vendors. The MCLPA""s from these vendors are available only in a power range of about 25 to 500 W.
The present invention seeks to provide a novel base station for cellular wireless communications based on a modular structure.
The present invention includes an active radiator module (ARM) that serves as a basic transmit/receive module in a variety of cellular base station configurations. The active radiator modules follow the trend of cellular architecture development and are designed to meet both current and future needs. It is a novel approach that may reduce the cost of the base station while providing desired flexibility.
In the active radiator module system, a combined signal is transmitted in low power through a cable to a mast, where it redistributes to the active radiator modules. The number of active radiator modules needed is a function of both the total effective radiated power (ERP) and gain required. The receive chain includes an LNA in each element, which reduces the noise figure of the system. The same active radiator module can serve in microcells that require small power and low gain antennas.
A remote RF unit is the least expensive solution for microcells. Its applicability is limited by the cost of RF trunking. It is the preferred solution for operators that have an access to the CATV or to fiber trunking already laid. This unit includes an amplifier, an LNA, and a transformer to the trunking band. This same module may be a part of a microcell or a picocell, but the RF is included inside the package, while the antenna is typically separate. The modular structure of the base station of the present invention provides readily upgradable base station performance at relatively low cost.
By way of example only, the present invention is described herein for certain commonly-used frequency ranges, such as for cellular telephones or PCS. However, it is appreciated that the present invention is not limited to these frequency ranges and may be applied to any set of frequencies.
There is thus provided in accordance with a preferred embodiment of the present invention, a modular cellular wireless communication base station including a plurality of active radiator modules located at a desired antenna location, each module including at least one antenna for transmitting and receiving, a transmitter including a power amplifier, and a receiver, a beam forming network controlling the relative amplitudes and phases of each of the modules, and an RF front end transmitting over a low power link with the plurality of active radiator modules via the beam forming network and receiving over a lower power link via a low noise amplifier.
In accordance with a preferred embodiment of the present invention, the RF front end is located remote from the plurality of modules. Preferably each module is self-enclosed.
Additionally in accordance with a preferred embodiment of the present invention at least one of the active radiator modules comprise two separate transmit and receive antenna elements. Preferably the transmit and receive antenna elements are isolated from each other by about 15-30 dB, most preferably by about 20 dB.
Further in accordance with a preferred embodiment of the present invention the beam forming network is located adjacent the plurality of active radiator modules, one for transmit and one for receive.
Still further in accordance with a preferred embodiment of the present invention, the modular cellular wireless communication base station includes a CATV up/down converter module. Preferably the CATV up/down converter module comprises a coaxial cable connected to a CATV network, the cable carrying a CATV forward link and reverse link. A CATV diplexer is preferably provided that separates transmit and receive signals. The converter module preferably comprises a mixer, a phased locked oscillator and a band pass filter, thereby to eliminate image and low frequencies.
In accordance with a preferred embodiment of the present invention the RF front end communicates with the beam forming network via a fiber optic link. In one embodiment, at least two separate fibers separately carry transmitter and receiver signals. Alternatively, one fiber carries both transmitter and receiver signals, and a splitter and a filter are provided to split and filter the signals.
Additionally in accordance with a preferred embodiment of the present invention, the transmitter amplifier comprises a first stage comprising a monolithic silicon gain stage and a second stage comprising a hybrid packaged power amplifier.
Further in accordance with a preferred embodiment of the present invention a transmitter filter is provided that reduces transmitter wide band noise in a receiver band. Additionally or attentively, a transmitter filter reduces spurious signals that interfere with a receiver channel of a cell.
Still further in accordance with a preferred embodiment of the present invention, there are provided a receiver amplifier and a receiver filter, wherein the receiver filter reduces a transmitter signal to a level wherein interfering intermod products are not generated in the receive chain, and the receive-amplifier is not desensitized by saturation. The other purpose of the receiver filter is to reduce interfering signals from other base stations and other systems.
Yet further in accordance with a preferred embodiment of the present invention, a receiver filter is provided that reduces interfering signals from sources external to the wireless communication base station.
In accordance with a preferred embodiment of the present invention, the plurality of active radiator modules are stacked to form an active antenna having desired gain and beam shape determined by the beam forming network. The modules may be stacked in a vertical array, a planar array or a circular array, for example.
Furthermore, in accordance with a preferred embodiment of the present invention, the active radiator modules include one transmit antenna and first and second receive antenna elements. The single transmit antenna is a vertically polarized antenna, the first receive antenna is polarized at +45xc2x0 and the second receive antenna is polarized at xe2x88x9245xc2x0.
Furthermore, in accordance with a preferred embodiment of the present invention, the plurality of active radiator modules are configured for a width less than 0.7 wavelengths, for forming a multitude of beams in the horizontal plane. The active radiator modules are configured for a height less than 1 wavelength, for forming a broad side radiation from a vertically stacked column of the plurality of active radiator modules.
Furthermore, In an alternative embodiment, in accordance with a preferred embodiment of the present invention, the active radiator modules include two transmit antennas and one receive antenna element. The receive antenna is a vertically polarized antenna and the first transmit antenna is polarized at +45xc2x0 and the second transmit antenna is polarized at xe2x88x9245xc2x0.
Additionally, in accordance with a preferred embodiment of the present invention, the modular cellular wireless communication base station further includes a transmit amplifier coupled to the transmit antenna and a receive amplifier coupled to receive antenna elements.
There is also provided in accordance with a preferred embodiment of the present invention, a method for mitigating a fading of signals on a forward link of a CDMA wireless system, the method including splitting a transmission signal to a plurality of transmitter antennas, introducing a delay that is longer than a CDMA chip in a transmit chain of the antennas relative to a first of the antennas, transmitting the signals by all the antennas, receiving the signals with different correlators, and combining the signals, thereby mitigating a fading of the signals.
Preferably each the antenna transmits with approximately equal coverage.
In accordance with a preferred embodiment of the present invention, the step of transmitting comprises transmitting from a plurality of spaced antennas.
Additionally in accordance with a preferred embodiment of the present invention the step of transmitting comprises transmitting from a plurality of antennas that transmit at different polarization.
Further in accordance with a preferred embodiment of the present invention the step of combining comprises combining with natural multipath signals.
There is also provided in accordance with a preferred embodiment of the present invention, a modular dual polarized base station antenna system including a plurality of pairs of orthogonal polarization antennas, wherein one of the pairs is polarized at xc2x145xc2x0 and another of the pairs is H-V polarized. Preferably a pair of transmit antennas are polarized at xc2x145xc2x0, and a pair of receive antennas are H-V polarized. Alternatively all pairs of antennas may be H-V polarized. Preferably each antenna is fed by a separate amplifier.
In accordance with a preferred embodiment of the present invention at least one isolation structure is provided for increasing isolation between the antenna pairs.
There is also provided in accordance with a preferred embodiment of the present invention, a method for modular dual polarized base station transmission and reception, the method including transmitting with a pair of transmit antennas polarized at xc2x145xc2x0, and receiving with a pair of receive antennas that are H-V polarized. Alternatively all pairs of antennas may be H-V polarized.
In accordance with a preferred embodiment of the present invention the transmit signals are split and weights of polarization are applied at a base station. Alternatively, weights of polarization are applied by control of amplifier gain. The weights may be applied at RF, IF or baseband frequencies.